1. Field of the Invention
The present invention relates to a data reproducing method using tilt adjustment to a high density recording optical disc and a data reproducing apparatus for the same.
2. Description of the Related Art
In an optical disc apparatus, data is recorded on an optical disc and the data is read out from the optical disc, by using an optical head. In such an optical disc apparatus, a tilt of the optical head to the optical disc has a large influence on the performance of the optical disc apparatus. If there is not any tilt between the optical disc and the optical head, a laser beam is well irradiated to the data recording surface of the optical disc and good record reproduction is carried out. However, when there is a tilt between the optical disc and the optical head, an aberration is caused in the laser beam irradiated to the data recording surface of the optical disc, resulting in degradation of the record reproduction performance.
In the tilt are generally a radial tilt as a tilt of the optical head in a radial direction from a perpendicular direction to the data recording surface of the optical disc, and a tangential tilt as a tilt of the optical head in a perpendicular direction to the radial direction from the perpendicular direction to the data recording surface of the optical disc.
FIG. 2 shows a relation between error rate and radial tilt. It could be seen from FIG. 2 that the error rate increases when the radial tilt becomes large. That is, this shows that the record reproduction performance degrades remarkably when the tilt is caused. Therefore, various methods are proposed of correcting the tilt to improve the reproduction performance.
In Japanese Laid Open Patent example Application (JP-P2003-157553A: first conventional example), one main beam and two sub beams are irradiated to the optical disc, and a tilt is detected based on detection results of these beams. An optical disc or an optical head is controlled based on a result of the tilt detection so as to cancel the tilt.
Also, in Japanese Laid Open Patent Application (JP-P2003-16677A: second conventional example), a 4-divided photo detector is used. A radial tilt is detected based on light quantity balance between two detection areas of the detector divided a line in a movement direction of the optical head and a tangential tilt is detected based on a light quantity balance between two detection areas of the detector divided by a line perpendicular to the movement direction of the optical head. The tilt control is carried out based on the detected tilt.
Also, in Japanese Laid Open Patent Application (JP-P2001-256652A: third conventional example), a tilt control is carried out through a signal process without any mechanical tilt control by changing a rate of outputs from two detection areas of a detector divided by a line in a movement direction of the optical head in accordance with the tilt.
Also, in Japanese Laid Open Patent Application (JP-A-Heisei 09-054953: fourth conventional example), three or five beams are irradiated to an optical disc, and a tilt is detected by using a reflection intensity of each beam. One of these beams which has the most excellent reproduction quality is selected for data reproduction.
Also, in Japanese Laid Open Patent Application (JP-P2000-149298A: fifth conventional example), DPP and DPD signals are generated by using detection signals of a main spot and side spots or detection signals of the side spots. A tilt angle of the optical disc is detected based on a difference calculation value between both of these signals.
However, a plurality of beams must be irradiated and the optical head becomes complicated in the first, fourth and fifth conventional examples. In addition, the tilt detection signal is obtained without any direct relation to the recorded signal or reproduction signal. Therefore, there would be a case where the tilt control is not carried to optimize the reproduction performance.
Also, in the fourth conventional example, an output of either of the three or five beams is used as a reproduction signal. The beam intensity of each beam must be large from the viewpoint to signal noise ratio (SNR), which requires a laser diode with a very high output. Therefore, the fourth conventional example is not in practical.
The second conventional example has a relatively simple structure. However, the tilt detection signal with enough precision cannot be obtained actually in such tilt detection.
The third conventional example has a relatively simple structure, and the tilt control is carried out by changing the output rate of the divided detection areas. However, the largest problem is in that an optimal output rate must be determined while a jitter or error rate as a parameter is measured (the error rate indicates a difference between a recorded bit string and the reproduced and decoded bit string and calculated from (error bits)/(a total of bits)). The jitter and the error rate do not have polarity. When such a parameter is used for the tilt control, a correction direction of the tilt cannot be immediately determined. Therefore, the tilt control is actually carried out while the direction of the tilt correction is changed in a positive or negative direction to find the optimal position. That is, the tilt control is carried out by using a so-called mountain climbing method. This requires a long learning time, and is very large performance degradation in the optical disc apparatus. In the third conventional example, it is described that a correction quantity of the output rate may be previously stored. However, the so-called mountain climbing method is needed to previously obtain the correction quantity. Also, the tilt changes depending on the temperature and humidity of the optical disc apparatus and has a time-based change. Therefore, sufficient precision could not be expected by the previously stored correction quantity.
Also, in most of the conventional tilt detection methods which are known at present, tilt detection is carried out by using a signal different from an actual reproduction signal. Therefore, an optimal tilt point for the reproduction signal and an optimal tilt point obtained from the tilt detection signal are often different. Therefore, It is not possible to say that the conventional tilt detection methods have a sufficient detection precision.
In conjunction with the above description, a tile detecting method is disclosed in Japanese Laid Open Patent application (JP-P-2002-56556A). In this conventional example, a light receiving section receives diffraction light reflected from a recording medium. A tile quantity of the recording medium is detected based on light intensity in interference region of the 0-th order diffraction light and one of the diffraction light components other than the 0-th order diffraction light of the received reflection light.